JP2014229808A - Avalanche photodiode - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable a high-frequency electrical signal to be extracted with lower image multiplication factor.SOLUTION: An avalanche photodiode comprises a first field control layer 105 and a second field control layer 106, which are formed between a second contact layer 103 and an avalanche layer 104, and composed of group III-V compound semiconductor, and have a second conductivity type. The second field control layer 106 has smaller bandgap energy in comparison with the first field control layer 105.

Description

  The present invention relates to an avalanche photodiode using an avalanche phenomenon.

  An avalanche photodiode (APD) is a device used as an optical receiver with low noise by amplifying the number of carriers generated by light absorption by an avalanche multiplication mechanism. A recent long-wavelength avalanche photodiode generally has a SAM (Separeted Absorption and Multiplication) structure in which an avalanche layer (multiplication layer) is separated from a light absorption layer.

  As described above, the greatest feature of the avalanche photodiode is to obtain a high receiving sensitivity as compared with a general PIN-photodiode. However, when an avalanche photodiode is used for actual optical communication, a parameter called maximum reception sensitivity is extremely important as well as minimum reception sensitivity. This is because in actual optical communication, depending on the communication method and communication distance, strong signal light of 0 dBm or more may be incident from extremely weak signal light of −20 dBm or less.

  In general, an avalanche photodiode can obtain a high current value due to multiplication even for a constant light intensity, and therefore has low resistance to strong light compared to a general PIN-photodiode. Therefore, when receiving a small amount of signal light, the bias voltage of the avalanche photodiode is set high to increase the avalanche photodiode's multiplication effect, and when receiving a strong signal light, the avalanche photodiode It is necessary to use a diode with a low bias voltage and a low multiplication factor.

E. Ishimura et al., "Degradation Mode Analysis on Highly Reliable Guardring-Free Planar InAlAs Avalanche Photodiodes", JOURNAL OF LIGHTWAVE TECHNOLOGY, vol.25, no.12, pp.3683-3693, 2007. Y. Hirota, Y. Muramoto, T. Takeshita, T. Ito, H. Ito, S. Ando and T. Ishibashi, "Reliable non-Zn-diffused InP = InGaAs avalanche photodiode with buried n-InP layer operated by electron injection mode ", ELECTRONICS LETTERS 14th, vol.40, no.21, 2004. Masahiro Nada, Yoshifumi Muramoto, Haruki Yokoyama, Naoteru Shigekawa, Tadao Ishibashi, and Satoshi Kodama, "Inverted InAlAs / InGaAs Avalanche Photodiode with Low.High.Low Electric Field Profile", Japanese Journal of Applied Physics, vol.51,02BG03, 2012 . Y. Muramoto and T. Ishibashi, "InP = InGaAs pin photodiode structure maximising bandwidth and efficiency", ELECTRONICS LETTERS 27th, vol.39, no.24, 2003.

  However, in an avalanche photodiode, even in a high multiplication operation, an avalanche breakdown or a zener breakdown does not occur in a light absorption layer made of InGaAs or the like, and a breakdown occurs only in an avalanche layer. An electric field control layer is provided between the light absorption layer and the avalanche layer to optimize the electric field strength of both. For this reason, in the avalanche photodiode, no photocurrent is detected from the time of bias application to several volts. A voltage value at which the photocurrent starts to be detected is called an on-voltage.

  The on-voltage is generally a voltage at which an electric field starts to be applied to the light absorption layer. At this time, a certain electric field strength has already been generated in the avalanche layer. Therefore, a constant multiplication factor has already been reached at the on-voltage.

  Furthermore, a band offset due to a difference in band gap exists between InGaAs generally used as a light absorption layer and InAlAs, InP or the like generally used as an avalanche layer. For this reason, even at the on-voltage, the electric signal generated by the high-frequency optical signal is hindered by the band offset described above and cannot be extracted as a high-frequency electric signal. The voltage that can be extracted as a high-frequency electrical signal is a few volts higher than the on-voltage, and therefore the multiplication factor is also higher.

  Thus, in order to obtain a higher maximum receiving sensitivity in an avalanche photodiode, it is important how a high-frequency electric signal can be extracted with a low multiplication factor.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to enable extraction of a high-frequency electric signal at a lower multiplication factor.

  The avalanche photodiode according to the present invention includes a first conductivity type first contact layer made of a III-V group compound semiconductor and an undoped avalanche layer made of a group III-V compound semiconductor formed on the first contact layer. A plurality of second conductivity type electric field control layers made of a III-V compound semiconductor formed on the avalanche layer and having different band gap energies, and of the plurality of electric field control layers, opposite to the first contact layer A light-absorbing layer made of a III-V group compound semiconductor formed on the electric field control layer located in the region, and a second conductivity type second contact made of a III-V group compound semiconductor formed on the light-absorbing layer. And the plurality of electric field control layers have smaller band gap energy closer to the light absorption layer.

  The avalanche photodiode may include a band continuous layer formed of a group III-V compound semiconductor formed between adjacent electric field control layers and having a band gap energy between two adjacent electric field control layers. Good. Further, the band continuous layer may be configured such that the band gap energy continuously changes in the layer thickness direction.

  As described above, according to the present invention, it is possible to obtain an excellent effect that a high-frequency electric signal can be extracted with a lower multiplication factor.

FIG. 1 is a configuration diagram showing the configuration of the avalanche photodiode according to the first embodiment of the present invention. FIG. 2 is a band diagram showing a change in band gap energy in the stacking direction of each layer of the avalanche photodiode in the first embodiment of the present invention. FIG. 3 is a configuration diagram showing the configuration of the avalanche photodiode according to the second embodiment of the present invention. FIG. 4 is a band diagram showing a change in band gap energy in the stacking direction of each layer of the avalanche photodiode according to the second embodiment of the present invention. FIG. 5 is a configuration diagram showing the configuration of the avalanche photodiode according to the third embodiment of the present invention. FIG. 6 is a band diagram showing a change in band gap energy in the stacking direction of each layer of the avalanche photodiode according to the third embodiment of the present invention.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

[Embodiment 1]
First, Embodiment 1 of the present invention will be described with reference to FIGS. FIG. 1 is a configuration diagram showing the configuration of the avalanche photodiode according to the first embodiment of the present invention. FIG. 1 schematically shows a cross section. FIG. 2 is a band diagram showing a change in band gap energy in the stacking direction of each layer of the avalanche photodiode in the first embodiment.

  In the avalanche photodiode, first, a first conductivity type first contact layer 102 made of a III-V group compound semiconductor formed on a substrate 101 and a III-V formed on the first contact layer 102 are formed. And an undoped avalanche layer 103 made of a group compound semiconductor.

  In addition, a plurality of second conductivity type electric field control layers made of a III-V compound semiconductor formed on the avalanche layer 103 and having different band gap energies, and the first contact layer 102 among the plurality of electric field control layers. And a light absorption layer 106 made of a group III-V compound semiconductor formed on the electric field control layer positioned opposite to the electrode. In Embodiment 1, a case where the first electric field control layer 104 and the second electric field control layer 105 are provided will be described as an example. Therefore, the light absorption layer 106 is formed on the second electric field control layer 105.

  In addition, the avalanche photodiode in the first embodiment includes a second contact layer 107 of the second conductivity type made of a III-V group compound semiconductor formed on the light absorption layer 106. In the first embodiment, the band continuous layer 108 is provided between the first electric field control layer 104 and the second electric field control layer 105.

  In the first embodiment, the first contact layer 102, the avalanche layer 103, the first electric field control layer 104, the band continuous layer 108, the second electric field control layer 105, the light absorption layer 107, and the second contact are formed on the substrate 101. The layer 107 is laminated in this order. The first electrode 111 is connected to the second contact layer 107, and the second electrode 112 is connected to the first contact layer 102.

  Note that it is important that each band gap energy of the plurality of electric field control layers is not less than the band gap energy of the light absorption layer or more and not more than the band gap energy of the avalanche layer 103. Here, the plurality of electric field control layers (the first electric field control layer 104 and the second electric field control layer 105) have smaller band gap energy as they are closer to the light absorption layer 106. In the first embodiment, the second electric field control layer 105 has a smaller band gap energy than the first electric field control layer 104.

  Here, the first electric field control layer 104 is disposed on the avalanche layer 103 side, and may have a band gap energy close to (same as) the avalanche layer 103. Similarly, the second electric field control layer 105 is disposed on the light absorption layer 107 side, and may have a band gap energy close to (same as) the light absorption layer 107.

  The band continuous layer 108 has a band gap energy between the adjacent first electric field control layer 104 and the second electric field control layer 105. Here, when the difference in band gap energy between the first electric field control layer 104 and the second electric field control layer 105 is not so large, the band continuous layer 108 may not be used. In the first embodiment, the first conductivity type is n-type and the second conductivity type is p-type.

  For example, the substrate 101 may be a semiconductor substrate made of semi-insulating InP that has been made high resistance by doping iron. The first contact layer 102 only needs to be made of n-type InP into which impurities are introduced at a high concentration. Moreover, the avalanche layer 103 should just be comprised from undoped InAlAs. InAlAs is a material whose electron ionization rate is higher than the hole ionization rate.

  The first electric field control layer 104 only needs to be made of p-type InAlAs. Moreover, the band continuous layer 108 should just be comprised from InAlGaAs of the composition whose band gap energy is 1.1 eV. The second electric field control layer 105 only needs to be made of p-type InGaAs.

  Moreover, the light absorption layer 107 should just be comprised from InGaAs. The second contact layer 107 only needs to be made of p-type InAlGaAs doped with impurities at a high concentration.

  Note that the avalanche layer 103, the first electric field control layer 104, the band continuous layer 108, the second electric field control layer 105, the light absorption layer 107, and the second contact layer 107 are patterned into a desired shape. It is a mesa structure. For example, the avalanche layer 103, the first electric field control layer 104, the band continuous layer 108, the second electric field control layer 105, the light absorption layer 107, and the second contact layer 107 are processed into a cylindrical shape having a diameter of about 22 μm. The side surface (side wall) of the mesa is protected by a passivation film (not shown) such as silicon nitride. Although not shown in the drawing, the first electrode 111 and the second electrode 112 are each connected to a lead-out wiring so that a potential can be applied.

  Here, the avalanche photodiode in Embodiment Mode 1 is a so-called bottom incidence type, in which light incident from the substrate 101 side is absorbed by the light absorption layer 107.

  Next, a method for manufacturing the avalanche photodiode described above will be briefly described. First, n-type InP (first contact layer 102), undoped InAlAs (avalanche layer 103), p-type InAlAs (first electric field control layer 104), undoped on a substrate 101 made of semi-insulating InP. InAlGaAs (band continuous layer 108), p-type InGaAs (second electric field control layer 105), undoped InGaAs (light absorption layer 107), and p-type InAlGaAs (second contact layer 107) are sequentially deposited by epitaxial growth. These may be formed by a well-known MOVPE method.

  Next, the first electrode 111 is formed on the p-type InAlGaAs layer. For example, a resist mask pattern having an opening is formed in a region to be the first electrode 111, and a three-layer laminated film of titanium layer / platinum layer / gold layer is formed thereon by electron beam evaporation. Thereafter, if the resist mask pattern is removed, the first electrode 111 that is in ohmic contact with the p-type InGaAsP layer (second contact layer 107) can be formed. This is a manufacturing method called a so-called lift-off method.

  Next, the layers of undoped InAlAs, p-type InAlAs, undoped InAlGaAs, p-type InGaAs, undoped InGaAs, and p-type InGaAsP are patterned by a known lithography technique and etching technique (wet etching). The avalanche layer 103, the first electric field control layer 104, the band continuous layer 108, the second electric field control layer 105, the light absorption layer 107, and the second contact layer 107 having the shape (mesa shape) are formed.

  Thereafter, the second electrode 112 is formed on the first contact layer 102 exposed by the patterning. The second electrode 112 has a three-layer structure of titanium layer / platinum layer / gold layer. Similar to the first electrode 111, the second electrode 112 may be formed by an electron beam evaporation method and a lift-off method.

  Next, the operation of the avalanche photodiode of Embodiment 1 will be described with reference to the band diagram of FIG. First, as described above, the avalanche photodiode according to the first embodiment is an electron injection avalanche photodiode in which the avalanche layer 103 is made of InAlAs whose electron ionization rate is higher than the hole ionization rate.

  For this reason, as the voltage is applied to the avalanche photodiode, the electric field strength in the avalanche layer 103 increases and depletion of the first electric field control layer 104 proceeds first. Next, when the depletion of the first electric field control layer 104 is completed, the electric field strength in the band continuous layer 108 increases and the depletion of the second electric field control layer 105 proceeds. Thereafter, when the depletion of the second electric field control layer 105 is finally completed, an electric field starts to be applied to the light absorption layer 107, and the voltage at this time becomes the on voltage.

  Here, as shown in FIG. 2B, the electric field strengths in the second electric field control layer 105, the band continuous layer 108, and the first electric field control layer 104 are already sufficiently high at the time of the on-voltage. The injected photoelectrons are not easily affected by the conduction band offset from the second electric field control layer 105 to the avalanche layer 103. Therefore, according to the first embodiment, in the electron injection type avalanche photodiode, the difference from the on-voltage to the voltage at which a high-frequency electric signal can be extracted can be reduced, and high-frequency operation at a low multiplication factor is possible. To.

[Embodiment 2]
Next, a second embodiment of the present invention will be described with reference to FIGS. FIG. 3 is a configuration diagram showing the configuration of the avalanche photodiode according to the second embodiment of the present invention. FIG. 3 schematically shows a cross section. FIG. 4 is a band diagram showing a change in band gap energy in the stacking direction of each layer of the avalanche photodiode in the second embodiment.

  The avalanche photodiode includes a substrate 201 first. Further, the first contact layer 202 of the first conductivity type made of a III-V group compound semiconductor is provided. Further, an undoped avalanche layer 203 made of a group III-V compound semiconductor formed on the first contact layer 202 and an avalanche layer 203 from the first contact layer 202 side toward the substrate 201 side. A plurality of electric field control layers of a second conductivity type made of a formed III-V group compound semiconductor and having different band gap energies.

  In addition, a light absorption layer 206 made of a III-V group compound semiconductor is provided on the electric field control layer positioned opposite to the first contact layer 202 among the plurality of electric field control layers. In Embodiment 2, a case where the first electric field control layer 204 and the second electric field control layer 205 are provided will be described as an example. Therefore, the light absorption layer 206 is formed on the second electric field control layer 205.

  The avalanche photodiode according to the second embodiment includes a second conductivity type second contact layer 207 made of a III-V group compound semiconductor formed on the light absorption layer 206. In the second embodiment, the band continuous layer 208 is provided between the first electric field control layer 204 and the second electric field control layer 205.

  In the second embodiment, the second contact layer 207, the light absorption layer 207, the second electric field control layer 205, the band continuous layer 208, the first electric field control layer 204, the avalanche layer 203, and the first contact are formed on the substrate 201. The layer 202 is laminated in this order. The first electrode 211 is connected to the second contact layer 207, and the second electrode 212 is connected to the first contact layer 202.

  It is important that each band gap energy of the plurality of electric field control layers is equal to or higher than the band gap energy of the light absorption layer and equal to or lower than the band gap energy of the avalanche layer 203. Here, the plurality of electric field control layers (the first electric field control layer 204 and the second electric field control layer 205) have smaller band gap energy as they are closer to the light absorption layer 206. In the second embodiment, the second electric field control layer 205 has a smaller band gap energy than the first electric field control layer 204.

  Here, the first electric field control layer 204 is disposed on the avalanche layer 203 side, and may have a band gap energy close to (same as) the avalanche layer 203. Similarly, the second electric field control layer 205 is disposed on the light absorption layer 207 side, and may have a band gap energy close to (same as) the light absorption layer 207.

  The band continuous layer 208 has a band gap energy between the adjacent first electric field control layer 204 and the second electric field control layer 205. Here, when the difference in band gap energy between the first electric field control layer 204 and the second electric field control layer 205 is not so large, the band continuous layer 208 may not be used. In the second embodiment, the first conductivity type is p-type and the second conductivity type is n-type.

  For example, the substrate 201 may be a semiconductor substrate made of semi-insulating InP that has been made high resistance by doping iron. The second contact layer 207 only needs to be made of n-type InP doped with impurities at a high concentration. Moreover, the light absorption layer 206 should just be comprised from InGaAs.

  The second electric field control layer 205 may be made of n-type InGaAs. Moreover, the band continuous layer 208 should just be comprised from InGaAsP of the composition from which a band gap energy will be 1.0 eV. The first electric field control layer 204 only needs to be made of n-type InP. Moreover, the avalanche layer 203 should just be comprised from undoped InP. InP is a material whose hole ionization rate is higher than the electron ionization rate. The second contact layer 207 only needs to be made of p-type InAlGaAs doped with impurities at a high concentration.

  The second contact layer 207, the light absorption layer 206, the second electric field control layer 205, the band continuous layer 208, the first electric field control layer 204, the avalanche layer 203, and the first contact layer 202 are patterned into desired shapes. For example, it is a well-known mesa structure. For example, the second contact layer 207, the light absorption layer 206, the second electric field control layer 205, the band continuous layer 208, the first electric field control layer 204, the avalanche layer 203, and the first contact layer 202 have a cylindrical shape with a diameter of about 22 μm. Has been processed. Further, the side surface (side wall) of the mesa is protected by a passivation film (not shown) such as silicon nitride. Although not shown in the drawing, the first electrode 211 and the second electrode 212 are connected to lead wires, respectively, so that a potential can be applied.

  Next, a method for manufacturing the avalanche photodiode described above will be briefly described. First, on a substrate 201 made of semi-insulating InP, n-type InP (second contact layer 207), undoped InGaAs (light absorption layer 206), n-type InGaAs (second electric field control layer 205), InGaAsP (Band continuous layer 208), n-type InP (first electric field control layer 204), undoped InP (avalanche layer 203), and p-type InAlGaAs (second contact layer 207) are sequentially deposited by epitaxial growth. These may be formed by a well-known MOVPE method.

  Next, for example, a ring-shaped second electrode 212 is formed on the p-type InAlGaAs layer. For example, a resist mask pattern having an opening is formed in a region to be the second electrode 212, and a three-layer laminated film of titanium layer / platinum layer / gold layer is formed thereon by electron beam evaporation. Thereafter, if the resist mask pattern is removed, the second electrode 212 that is in ohmic contact with the p-type InAlGaAs layer (second contact layer 207) can be formed. This is a manufacturing method called a so-called lift-off method.

  Next, layers of n-type InP, undoped InGaAs, n-type InGaAs, InGaAsP, n-type InP, undoped InP, and p-type InAlGaAs are patterned by a known lithography technique and etching technique (wet etching). Then, the second contact layer 207, the light absorption layer 206, the second electric field control layer 205, the band continuous layer 208, the first electric field control layer 204, the avalanche layer 203, and the first contact having a desired shape (mesa shape). Layer 202 is formed.

  Thereafter, the first electrode 111 is formed on the second contact layer 207 exposed by the patterning. The first electrode 111 has a three-layer structure of titanium layer / platinum layer / gold layer. Similar to the first electrode 211, the first electrode 111 may be formed by an electron beam evaporation method and a lift-off method.

  Next, the operation of the avalanche photodiode of the second embodiment will be described with reference to the band diagram of FIG. First, as described above, the avalanche photodiode according to the second embodiment is a hole injection avalanche photodiode in which the avalanche layer 203 is made of InAlAs having a hole ionization rate higher than the electron ionization rate.

  For this reason, as the voltage is applied to the avalanche photodiode, the electric field strength in the avalanche layer 203 increases, and first, depletion of the first electric field control layer 204 proceeds. Next, when the depletion of the first electric field control layer 204 is completed, the electric field strength in the band continuous layer 208 increases and the depletion of the second electric field control layer 205 proceeds. Thereafter, when the depletion of the second electric field control layer 205 is finally completed, an electric field starts to be applied to the light absorption layer 206, and the voltage at this time becomes the on voltage.

  Here, as shown in FIG. 4B, the electric field strength in the second electric field control layer 205, the band continuous layer 208, and the first electric field control layer 204 is already sufficiently high at the time of the on-voltage. The injected photoholes are not easily affected by the valence band offset from the second electric field control layer 205 to the avalanche layer 27. Therefore, according to the second embodiment, in the hole injection type avalanche photodiode, the difference from the on voltage to the voltage at which a high frequency electric signal can be extracted can be reduced, and high frequency operation at a low multiplication factor can be achieved. to enable.

[Embodiment 3]
Next, Embodiment 3 of the present invention will be described with reference to FIGS. FIG. 5 is a configuration diagram showing the configuration of the avalanche photodiode according to the third embodiment of the present invention. FIG. 5 schematically shows a cross section. FIG. 6 is a band diagram showing a change in band gap energy in the stacking direction of each layer of the avalanche photodiode in the third embodiment.

  In the avalanche photodiode, first, a first conductivity type first contact layer 302 made of a group III-V compound semiconductor formed on a substrate 301 and a III-V formed on the first contact layer 302 are formed. And an undoped avalanche layer 303 made of a group compound semiconductor.

  Also, a plurality of second conductivity type electric field control layers made of a III-V group compound semiconductor formed on the avalanche layer 303 and having different band gap energies, and the first contact layer 302 among the plurality of electric field control layers. And a light absorption layer 306 made of a III-V group compound semiconductor formed on the electric field control layer positioned opposite to the electrode. In Embodiment 3, a case where the first electric field control layer 304 and the second electric field control layer 305 are provided will be described as an example. Therefore, the light absorption layer 306 is formed on the second electric field control layer 305.

  In addition, the avalanche photodiode in the third embodiment includes a second contact layer 307 of the second conductivity type made of a III-V group compound semiconductor formed on the light absorption layer 306. In Embodiment 3, the band continuous layer 308 is provided between the first electric field control layer 304 and the second electric field control layer 305.

  In Embodiment 3, the first contact layer 302, the avalanche layer 303, the first electric field control layer 304, the band continuous layer 308, the second electric field control layer 305, the light absorption layer 307, and the second contact are formed on the substrate 301. A layer 307 is stacked in this order. The first electrode 311 is connected to the second contact layer 307, and the second electrode 312 is connected to the first contact layer 302.

  Note that it is important that each band gap energy of the plurality of electric field control layers is equal to or higher than the band gap energy of the light absorption layer and equal to or lower than the band gap energy of the avalanche layer 303. Here, the plurality of electric field control layers (the first electric field control layer 304 and the second electric field control layer 305) have smaller band gap energy as they are closer to the light absorption layer 306. In the third embodiment, the second electric field control layer 305 has a smaller band gap energy than the first electric field control layer 304.

  Here, the first electric field control layer 304 is disposed on the avalanche layer 303 side, and may have a band gap energy close to (same as) the avalanche layer 303. Similarly, the second electric field control layer 305 is disposed on the light absorption layer 307 side, and may have a band gap energy close to (same as) the light absorption layer 307.

  The band continuous layer 308 has a band gap energy between the adjacent first electric field control layer 304 and the second electric field control layer 305. In addition, the band gap energy continuously changes in the layer thickness direction. Yes. In the band continuous layer 308, the band gap energy continuously changes from the band gap energy of the light absorption layer 306 (second electric field control layer 305) to the band gap energy of the avalanche layer 303 (first electric field control layer 304). Yes. In the third embodiment, the first conductivity type is n-type and the second conductivity type is p-type.

  For example, the substrate 301 may be a semiconductor substrate made of semi-insulating InP that has been made high resistance by doping iron. The first contact layer 302 only needs to be made of n-type InP into which impurities are introduced at a high concentration. The avalanche layer 303 only needs to be made of undoped InAlAs. InAlAs is a material whose electron ionization rate is higher than the hole ionization rate.

The first electric field control layer 304 only needs to be made of p-type InAlAs. Further, the band continuous layer 308 may be made of InAl _x Ga _1-x As having a composition in which _x continuously changes from 1 to 0 from the substrate 301 side to the light absorption layer 306 side. The second electric field control layer 305 only needs to be made of p-type InGaAs.

  Moreover, the light absorption layer 306 should just be comprised from InGaAs. The second contact layer 307 only needs to be made of p-type InAlGaAs doped with impurities at a high concentration.

  The avalanche layer 303, the first electric field control layer 304, the band continuous layer 308, the second electric field control layer 305, the light absorption layer 306, and the second contact layer 307 are patterned into desired shapes, for example, well-known. It is a mesa structure. For example, the avalanche layer 303, the first electric field control layer 304, the band continuous layer 308, the second electric field control layer 305, the light absorption layer 306, and the second contact layer 307 are processed into a cylindrical shape having a diameter of about 22 μm. Further, the side surface (side wall) of the mesa is protected by a passivation film (not shown) such as silicon nitride. Although not shown, the first electrode 311 and the second electrode 312 are each connected to a lead-out wiring so that a potential can be applied.

  Here, the avalanche photodiode according to Embodiment 3 is a so-called bottom-surface incident type, in which light incident from the substrate 301 side is absorbed by the light absorption layer 306.

Next, a method for manufacturing the avalanche photodiode described above will be briefly described. First, n-type InP (first contact layer 302), undoped InAlAs (avalanche layer 303), p-type InAlAs (first electric field control layer 304), undoped on a substrate 301 made of semi-insulating InP. InAl _x Ga _{1 -x} As (band continuous layer 308), p-type InGaAs (second electric field control layer 305), undoped InGaAs (light absorption layer 306), and p-type InAlGaAs (second contact layer 307) Sequentially deposited by epitaxial growth. These may be formed by a well-known MOVPE method.

  Next, the first electrode 311 is formed on the p-type InAlGaAs layer. For example, a resist mask pattern having an opening is formed in a region to be the first electrode 311, and a three-layer laminated film of titanium layer / platinum layer / gold layer is formed thereon by electron beam evaporation. Thereafter, if the resist mask pattern is removed, the first electrode 311 that is in ohmic contact with the p-type InGaAsP layer (second contact layer 307) can be formed. This is a manufacturing method called a so-called lift-off method.

  Next, the layers of undoped InAlAs, p-type InAlAs, undoped InAlGaAs, p-type InGaAs, undoped InGaAs, and p-type InGaAsP are patterned by a known lithography technique and etching technique (wet etching). The avalanche layer 303, the first electric field control layer 304, the band continuous layer 308, the second electric field control layer 305, the light absorption layer 306, and the second contact layer 307 are formed.

  Thereafter, a second electrode 312 is formed on the first contact layer 302 exposed by the patterning. The second electrode 312 has a three-layer structure of titanium layer / platinum layer / gold layer. Similar to the first electrode 311, the second electrode 312 may be formed by an electron beam evaporation method and a lift-off method.

  Next, the operation of the avalanche photodiode of Embodiment 3 will be described using the band diagram of FIG. First, as described above, the avalanche photodiode of the third embodiment is an electron injection type avalanche photodiode in which the avalanche layer 303 is made of InAlAs whose electron ionization rate is higher than the hole ionization rate.

  For this reason, as the voltage is applied to the avalanche photodiode, the electric field strength in the avalanche layer 303 increases, and first, depletion of the first electric field control layer 304 proceeds. Next, when the depletion of the first electric field control layer 304 is completed, the electric field strength in the band continuous layer 308 increases and the depletion of the second electric field control layer 305 proceeds. Thereafter, when the depletion of the second electric field control layer 305 is finally completed, an electric field starts to be applied to the light absorption layer 306, and the voltage at this time becomes the on voltage.

  Here, as shown in FIG. 6B, the electric field strength in the second electric field control layer 305, the band continuous layer 308, and the first electric field control layer 304 is already sufficiently high at the time of the on-voltage. The injected photoelectrons are not easily affected by the conduction band offset from the second electric field control layer 305 to the avalanche layer 303.

  In addition, in Embodiment 3, the band continuous layer 308 is a composition displacement layer, and the band of the band continuous layer 308 is continuously formed in the layer thickness direction from the band gap energy of the light absorption layer 306 to the band gap energy of the avalanche layer 303. The gap energy is changed. This configuration can further reduce the influence of the band offset.

  As described above, according to the third embodiment, in the electron injection type avalanche photodiode, the difference from the on-voltage to the voltage at which the high-frequency electrical signal can be extracted can be reduced, and the high-frequency at a low multiplication factor can be achieved. Enable operation.

  As described above, according to the present invention, a plurality of electric field control layers are provided between the light absorption layer and the avalanche layer, and the plurality of electric field control layers have smaller band gap energy as they are closer to the light absorption layer. As a result, a high-frequency electric signal can be extracted with a lower multiplication factor. With the above-described configuration, the electric field strength of the band offset portion existing between the light absorption layer and the avalanche layer can be locally increased, and the band of the portion where the offset exists is curved at the time of the on-voltage. . As a result, there is no band offset that hinders the high-frequency electrical signal at the on-voltage, so that the multiplication factor in the voltage at which the high-frequency electrical signal can be extracted can be reduced, and higher maximum receiving sensitivity can be realized.

  The present invention is not limited to the embodiment described above, and many modifications and combinations can be implemented by those having ordinary knowledge in the art within the technical idea of the present invention. It is obvious. In the above description, the case where two electric field control layers are used has been described as an example. However, the present invention is not limited to this, and the same applies even when three or more electric field control layers are used. In any case, it is sufficient that the band gap energy is smaller as it is closer to the second contact layer.

  In the above-described embodiment, an avalanche photodiode having a simple mesa structure is taken as an example, but the present invention is not limited to this. For example, the present invention can also be applied to an avalanche photodiode having a guard ring for preventing edge breakdown.

  The present invention is also applicable to an inverted avalanche photodiode that does not require Zn diffusion, Si ion implantation, or selective doping (see Non-Patent Documents 1, 2, and 3). Furthermore, although the case where the light absorption layer is made of undoped InGaAs has been described as an example, the present invention is not limited to this. For example, the present invention can also be applied to an avalanche photodiode having a so-called MIC absorption layer in which p-type / undoped types are stacked (see Non-Patent Document 4).

  Further, the electric field structure of the avalanche photodiode, which is applied to the inverting avalanche photodiode, has a second conductivity type layer on the opposite side of the electric field control layer with the avalanche layer interposed therebetween. “low” does not lose the generality of the present invention (see Non-Patent Document 3).

  DESCRIPTION OF SYMBOLS 101 ... Substrate, 102 ... First contact layer, 103 ... Avalanche layer, 104 ... First electric field control layer, 105 ... Second electric field control layer, 106 ... Light absorption layer, 107 ... Second contact layer, 108 ... Band continuous layer 111 ... 1st electrode, 112 ... 2nd electrode.

Claims (3)

A first contact layer of a first conductivity type made of a group III-V compound semiconductor;
An undoped avalanche layer made of a III-V compound semiconductor formed on the first contact layer;
A plurality of electric field control layers of a second conductivity type made of a III-V group compound semiconductor formed on the avalanche layer and having different band gap energies;
A light absorption layer made of a III-V group compound semiconductor formed on the electric field control layer positioned opposite to the first contact layer among the plurality of electric field control layers;
A second contact layer of a second conductivity type made of a III-V compound semiconductor formed on the light absorption layer, and
The plurality of electric field control layers have smaller band gap energy as they are closer to the light absorption layer. The avalanche photodiode according to claim 1,
An avalanche photodiode comprising a band continuous layer formed of a group III-V compound semiconductor formed between adjacent electric field control layers and having a band gap energy between two adjacent electric field control layers . The avalanche photodiode according to claim 2,
The band continuous energy of the band continuous layer continuously changes in the layer thickness direction.

Images